Vacuum tube



H. A. SNOW,

VAGUUMTUBE July 21, 1936.

Original Filed March 19, 1930 2 Sheets-Sheet l l -/5 [cy ran-s INVENTOR HAROLD A. SNOW BY /1 g QM ATTORNEY July 21, 1936. H. A. SNOW 2,048,231

VACUUM TUBE Original Filed. March 19, 1950 2 Sheets-Sheet 2 I I l I v I I I I I I /00 -60 -50 -40 "20 -10 [c Van's Mg 3 Ex 50. 40- F is 30 N R: 20- E g /0 s: ta 3" 0 ga I I I Q; 3- E a g I I I II I I l I llll I l I .2 .4 .5-81 2 3 4 6 8/0 .20 30 (WM/f1? 7017451 650 IfC. 3075/11001/[47'59 47' 6'0- INVENTOR HAROLD A. SNOW I aY/fglgw ATTORNEY Patented July 21, 1936 "*2 STATES PATENT OFFICE VACUUM TUBE Harold A. Snow, Mountain Lakes, N. J., assignor to Radio Corporation of America, a corporation of Delaware 10 Claims.

The present application relates to space discharge tubes of the type including a cathode, a control grid and anode, and is a division of my application, Serial No. 437,225, filed December The problem of preventing modulation distortion in an amplifier stage, or in cascaded stages, has imposed severe limitations upon the range of signal voltages which may be applied to the amplifier. In'radio receivers, for example, high sensitivity is desirable for the reception of weak signals, and some form of manual, or automatic, control must be provided to reduce the amplifier transmission, or gain, when stronger l5 signals are received. When a receiver of high sensitivity is operated in the vicinity of a broadcasting station, it is not unusual to find that the signal voltage applied to the first carrier wave amplifier is greater than the voltage required on the detector for normal output at the loud speaker. With the present types of electron discharge tubes, it is usual to adjust one of the operating potentials applied to the tube electrodes to decrease the amplification as the received signal strength increases.

1 Within the range of relatively low signal strengths, this reduction of amplification is not accompanied by modulation distortion, but wtih increasing signal strengths distortion is introduced when the amplification rate is adjusted to maintain an approximately constant output. Furthermore, within the range of higher signal strengths, it is frequently difficult to adjust the amplification to maintain constant output since the transconductance of the tube changes very rapidly for small changes in the transmission control voltage. This restricts the amplification control to a small range of applied control voltages, and, unfortunately, the rate of change of amplification is more gradual in the range of high amplification where a rapid change of amplification for small changes in control voltage would be permissible.

Cross-talk effects in radio frequency amplifiers depend upon the high-order curvature parameters of the tube, and are to that extent related to the problem of distortion discussed heretofore. It can be pointed out that the term cross-talk is employed to designate that species of interference which originates in the radio frequency amplifier tubes by modulation between two, or more, signals. The improvements discussed herein in connection with an electron discharge tube to reduce distortion will also reduce a large part of the cross-talk. Reference is made to the Proceedings of the Institute of Radio Engineers for December, 1930, wherein in an article entitled Reduction of distortion and cross-talk in radio receivers by means of variable-mu tetrodes there is demonstrated the intimate 5 relationship between the problems of distortion and cross-talk in radio frequency amplifiers, and their elimination by means of variable mu tubes.

It is, therefore, one of the main objects of the present invention to provide a space discharge 10 tube having such operating characteristics, that no distortion is introduced when, for increasing signal strengths, the operating potentials are so adjusted that the amplification rate is reduced to a small fraction of themaximum amplifica- 15 tion.

A further object of the invention is to provide an electron discharge tube having such characteristics that, when the potentials are adjusted to give a relatively low amplification of strong 20 signals, the change in transconductance for a given change in the gain control voltage is much lower than is the case with the known types of tubes.

Another, and important object of the inven- 5 tion is to provide a high frequency amplifier tube capable of adjustment to give an undistorted output of approximately constant magnitude over a wide range of applied carrier voltages.

A more specific object of the invention is to 30 provide an electronic amplifier in which different portions of the electron stream are influenced at different rates by the voltages applied to the control grid.

The novel features which I believe to be char- 35 acteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as-to both its organization and method of operation will best be understood by reference to the following description taken 40 in connection with the drawings in which I have indicated diagrammatically several arrangements whereby my invention may be carried into effect.

In the drawings, 45

Fig. 1 is a perspective view, partly in section, of a screen grid tube embodying the invention,

Fig. 2 is a diagrammatic view illustrating an embodiment of the invention,

Fig. 3 is a curve sheet showing the variations 50 of plate current with grid bias for a tube such as shown in Fig. 1,

v Fig. 4 is a curve sheet showing the relation between control grid voltage and transconductance for tubes embodying the invention, 55

Fig. 5 is a curve sheet showing the relation between permissible maximum input voltages and control grid bias voltages, and Fig. 6 is a curve sheet showing the performance of a three stage amplifier employing the novel form of tube. V

Referring to the accompanying drawings wherein like reference characters in the diifer p ent figures designate the same elements, the in vention is shown in Fig. 1 as embodied in a: form r of tube known commercially as a screen grid tube, the latter having a separate heater for the cathode. As is well known, this particular type of tube comprises an evacuated envelope enclosing a cathode C, heated; by a resistance (not; shown) within the cathode, an; inner grid'rCG, an. outer grid SG, a plate or anode P; and arr outer screen S which is electrically connected to the outer grid. Except for the novel construction of the control grid CG and its novel functional relationships to the remaining elements of" the tube, the several elements of the tube, andtheir relative physical arrangement, may be'substantially the sameas that-employed inthe present commercial tubes.

In tubes of' this general type, the control grid comprises a helicalwinding supported by one ormore wires l Inthis particular embodiment of the present invention, the helical winding is not continuous, as inthe-known constructions,

This particular construction results in a tube in whichthe control exercised upon the electron.

stream is not uniform over the entire exten thereof.

be explained as follows:

The control grid has been shown between them. At low negative biases the entire cathode isoperative, and the tube has about the same characteristics it would have if'the: gap were in place. As the grid bias increases negatively the'electron current through the upper and lower parts of the control grid are cut off leaving a low-mu control through the gap. At these bias voltages the tube acts as if. the

upper and lower sections of'the control grid were formed of solid meta1',.and controlledthe current through the gap in the ordinary manner;

In. Fig. 2. the cathodeC' takes. the form of a coating, represented by the stippl'edi section on the heaterelement. H- The plate F and screen grid SG are of the usual construction and arrangement, but one end of the. control grid vCG stops-short of the endof the cathode.

It; is; well knownthat the-"amplification of a vacuum tube may be regulated by' adjusting the bias voltage upon the: control grid,.theamplifi'cation decreasing asathe'biasvoltagebecomes morenegative; Curves showing. the relation between plate current and gridbias, i. a, transfer characteristics, afford 'an'in'di'cation ofth'e amplificationat difierent bias 'voltages, since the slope of the curve at any point isa" measureof the The operation of the tube shown inf Fig: 1 may to be divided into two sections which are mounted with a gap amplification when the tube is biased for operation at-that point. 7

In Fig. 3, the solid line curve A is the transfer characteristic for a tube'such as shown in Fig. 1, and the dotted line curve B is a similar curve for a commercial screen grid tube of the same general type but having .a continuous control grid Winding of uniform pitch. An examination of curve A shows that, with tubes embodying the invention, a control of amplification extends over a range of control grid bias of from. zero to more than 30 volts. With the known tubes, the curvature of the transfer characteristic. approaches' zero at a control grid bias of about '15.:volts.

In other words, an increase of the grid bias above approximately 15 volts negative will not be accompanied by a decrease in amplification Y whenv the known type of tube construction is employed, but with tubes embodying the invention, I the amplification 'may be varied with changes of control grid bias throughout'a range of from zero to upwardly of 30 volts. The tubes will still pass signals, by leakage transmission when the control grid biases exceed these respective values, but control of amplification is no longer possible in regions where the transconductance curves become substantially horizontal.

, The curves of Fig. 4 show the relation between. control grid-bias andtransconductance for two tubes embodying the invention, and for a similar tube which has the usual grid construction. Curve A is the transconductance-control grid bias 7 curve for a screen grid tube of the type shown in Fig. 1, having two turns omitted from the center of the control grid. The data for this curve and for curve A of Fig. 3 relates to the same tube. Curve A is a similar curvefor a screen grid tube in which only onetfurn' was removed from the center of the control gridand curve B shows the characteristic properties of the 7 conventional type of tube having a continuo control grid winding. 7

An examination of these curves shows that the useful range of" transmission control is considerably extended by the present invention. With the known-constructions, a decrease of the transconductance from about 500 micromhos to the value, about 0.8 micromho, at which leakage transmission prevents further amplification control, corresponds to a change in control grid bias of about ten volts. 'The corresponding ranges of control grid bias for the tubes of curves A and A. are, respectively, about and 60 volts- As stated'above, modulation distortion may ocour when, for a given signal strength, the 'amplification is so adjusted as to bring'the output down to a desired, or standard, level. Since suchdistortion is due to the curvature of the transfer characteristic'it will be'apparent that a tube having a curve of lower curvature can 7' transmit, without distortion, higher voltage signals'than a tube having a characteristic which exhibits a region of higher curvature. An examination of the curves of Fig. 3, will show that the maximum curvature of curve A is substantially lower than that of curve B.

Modulation distortion introduced by a tube may be determined by applying'a signal having a definite and constant modulation to the input of the tube, and measuring the modulation of the output signal. For small input signals the tube introduces practically no change in modulation.

When the input signal increases'beyond a cerv signal voltages which produce a twenty per cent rise in modulation. The twenty per cent rise in modulation was chosen as a standard as a matter of convenience since distortion of this magnitude may be observed by ear when the modulation is within the range of audible frequencies as is the case with speech or music. The data for curves A,'A', and B were obtained for the same tubes as those whose characteristic curves are identified by corresponding characters in Fig. 4, the signal in each instance being an 850 kilocycle carrier,

modulated 30% at 60 cycles.

In the case of the commercial tube, curves B of Figs. 4 and 5 show that over the range of bias voltages which control the amplification, i. e., from zero to about twelve volts negative, the

maximum input signal voltage which can be transmitted with not more than 20% distortion is about5 volts.

When the bias is adjusted to maintain a constant output signal, the maximum input signal "boltage that can be applied to the tube with less than 20% distortion is about 0.3 volts, corresponding to'a control grid bias of approximately 12 volts negative. For greater signal strengths, the

I increases of control grid bias do not alter the amplification, but do affect the maximum signal strength which may be transmitted with less than 20% distortion. For a single tube, signal strengths falling outside the range of volume control cannot be handled by the amplifier if a constant output is essential, but in cascaded amplifiers having two or more controlled stages, the maximum voltages, as shown by the dotted line portion of curve B, may be transmitted by the first stage when amplification control without additional distortion is provided in a subsequent stage.

r A similar analysis of curve A will show that r the maximum signal strength which may, without undue distortion, be transmitted to give the desired constant output voltage is about 1.1 volts, corresponding to a control grid bias of about 38 volts. For the tube with two turns removed from the control grid, curve A shows a permissible input voltage of 3 volts, with a bias of volts. Furthermore, by so restricting the control grid bias voltages that the maximum can never exceed about 28 volts negative in the case of the tube of curve A, and about 65 volts in the case of the tube of curve A, the maximum carrier voltages which may be transmitted are approximately 7 and 22 volts, respectively.

I The observations have been verified by tests made with a commercial radio receiver having three radio frequency amplifier stages employing commercial 224 tubes were used in the amplifier.-

To eliminate distortion in the third radio frequency stage, only the first two stages were adjusted to control the amplification. The distortion'was of the same values whether the bias on the control grid of the first tube was varied, or the bias control was extended to include both the first and second tubes. From curve D it will be noted that the modulation rise begins at a carrier input of about 0.15 volts, and reaches 20% at 0.3 signal voltage on the first tube.

Curve E was plotted from data obtained with the same radio receiver when tubes embodying the invention were substituted in the radio frequency stages. The tubes were of the type shown in Fig. 1, i. e., of standard 224 type construction except that two turns were omitted from the center of the control grid. With increasing signal' strength, the detector input was maintained constant by adjusting the control grid bias simultaneously' on the three amplifier stages. It is to be noted that the carrier input across the first tube increased to 10 volts before a modulation rise was apparent, and that it reached 17 volts before the modulation rise reached 20%. The variation of control grid bias with input signal strength is shown by curve F.

These results were checked qualitatively by a listening test with ordinary broadcast (music) modulation. It was found that the distortion on high modulation peaks became apparent when the input on the first amplifier was from 15 to 20 volts, becoming worse as the input was increased beyond 20 volts.

These observations of actual performance in a receiver check closely with the results plotted in Figs. 4 and 5 for single stages. By employing tubes constructed in accordance with the invention, the permissible input voltage was raised from 0.3 volts to 17 volts, i. e., volume control with good quality reproduction can be had with input voltages about 57 times as great as those which may be applied when the known commercial form of tube is employed.

It is to be understood that the invention is not limited to any particular type of tube, but is, in general, applicable to all tubes employed for amplification control. The physical construction of the control grid, or the geometric and structural relationships of the tube elements are subject to wide variation so long as the control grid exercises different rates of control at different portions of the electron stream. Considered broadly, the invention provides in a single tube. the electrical equivalent of two, or more, amplifier tubes operating in parallel, one tube having a relatively high ratio of, plate voltage to control grid voltage ,(high mu), one, or more, of the remaining tubes having lower ratios of plate voltage to control grid voltage (low mu). In fact, the same distortionless amplification control and substantial reduction of cross-talk over a wide range of input voltages may be secured when two or more tubes of the described difierent charac-,

teristics are operated in paralleL- A single tube exhibiting these characteristicslwill usuallybe more economical and convenient than the parallel tube arrangement. V V 3 Although the above discussion has been limited to a consideration of modulation distortion in radio receivers, it will be apparent that the curva- V ture' of the transfer characteristic givesrise to other forms of distortion which limit the range of continuous wave and audio frequency voltages the ratio of the change in one. electrode voltage to a change in the other electrode voltage, under the condition that a specified current remains unchanged.

While I have indicated and described several systems for carrying my invention'into eifect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular constructions shown and-described, but that many modifications may be made without departing from the "scope of my invention as set forth in the appended claims.

What I claim is:

1. An electron discharge tube comprising a cathode with an elongated electron emitting surface having substantially uniform electron emission per unit length throughout its normally:

active length, a plate-electrode surrounding said surface of said cathode throughout its length; and control means for the entire space discharge between said cathode and said plate consisting solely of a helical control grid of'uniform pitch and shorter than said surface of said cathode and positioned between said plate and said cathode to surround the major portion of said surface of said cathode with an unobstructed gap in registry with a minor portion of said surface of said cathode. r r

2. An electron discharge tube comprising an elongated cathode having substantially uniform electron emission per unit length throughout its active length at normal operating voltage, a control grid constituting the sole grid electrode immediately adjacent said cathode and surrounding said cathode and positioned'to cover the major part of said cathode at one end and leave a minor part of said cathode at the other'end uncovered, and a cold electrode surrounding said control grid and coextensive with said cathode to register with both' said covered and uncovered parts of said cathode.

3. An electron discharge tube comprising an elongated cathode having a surface of'substantially uniform electron emission per unit length throughout its active length at normal operating voltage, a coldelectrode surrounding and co-extensive with said active length of said cathode, and a control grid surrounding the active length of said cathode and being the only grid electrode in the path of the electron stream between said cathode and said cold electrode said grid electrode being positioned to extend along and'cover' said activelength of said cathode fromone end thereof and terminating considerably short' 01' the other end of'said cathode to leave a substan: tial portion ofsaid active length of said cathode directly exposed to said cold electrode. v r

,4. An electron discharge tube comprising a1 cathode with an elongated, electronemittlng sur face having substantially uniform electron emise. sion per unit length throughout its normally all ,of said active length of said cathode; and

space discharge control'means consisting ,ofga

part of said activelength at amend and leave an unobstructed space discharge path between said cold electrodeand al'substantial portion of V the-active length of said cathode at the other end.;

5. An electron discharge tube comprising an elongated equipotential cathode having through out its length substantially uniform electron emis: sion per unit length; a coldelectrode surrounding said cathode throughout its length, and control means for the space discharge between said oath ode and said cold velectrode'and consisting solelyof a helical control grid of uniform *pitch and substantially shorter than said cathode surrounding said cathode and extending between said .cold' electrode and said cathode with one end in regis-- try with one end of said cathode. 5 V

6. Anelectron discharge device having a vari-' able mu factor andcomprising an equi-potential cathode, a plate, a screen grid electrode coextensive with said cathode and interposed between said cathode and said plate, and means interposed between said cathode and said'screen' grid for. controlling the space discharge between. said cathode and said screen gridand consisting solely of a unipotential grid electrode, positionedbetween said cathode and said screen grid adjacent 45 rounding said screen grid, and a grid electrode shorter than said cathode and interposed between sole control electrode for the space discharge from said cathode to said screen grid and positioned to a cover the major portion of the-length of said cathode from one end to an intermediate point and leave the minor part of said cathode uncovered to said screen grid. a '8. An electron discharge device having a variable mu factor and comprising a straight equip'otential cathode, a tubular plate coaxial with and surrounding said cathode, a tubular grid electrode coaxial and coextensive with said cathodeand'in terposed between said cathode and said plate to surround said cathode, and a tubular; equipo ten-- tial grid electrode coaxial with and surrounding said cathode and interposed betweenthe major portion ofthe surface of said cathode and said other grid electrode and leaving an unobstructed space discharge path between the remainder of said cathode and. saidothergrid electrode.

10 active length, a ,cold electrode surrounding the active length-of said cathode and in registry with V f said cathode and said anode and constituting the 55 9. An electron discharge tube comprising an elongated cathode having an active surface with substantially uniform electron emission per unit length at normal operating voltage, a screen grid electrode in operative relation to all of the active surface of said cathode, an anode surrounding said screen grid, and control means for the space discharge between said cathode and said screen grid consisting solely of a unipotential helical grid electrode with a plurality of turns covering the major portion of the length of said cathode and spaced for a predetermined amount of control of said space discharge and positioned to leave uncovered a minor portion of the length of said cathode to provide an unobstructed gap for space discharge between said minor portion of said cathode and said screen grid.

10. An electron discharge tube comprising an elongated cathode having an active surface with substantially uniform electron emission per unit length at normal operating voltage, a cold electrode in operative relation to all of the active surface of said cathode, and control means consisting solely of a grid electrode interposed between said cathode and said cold electrode and constructed to cover the major portion of the active surface of said cathode and to leave a. minor portion of said surface uncovered and exposed directly to said cold electrode through an unobstructed space discharge path.

HAROLD A. SNOW. 

